In recent years with the increase of demands for semiconductors, functional ceramics and the like, high-temperature heat treatments have been increasingly carried out employing a vacuum furnace, a furnace for growth of monocrystals for semiconductors, a furnace for sintering ceramics, a kiln for calcining C/C composites or the like. Such high-temperature heat treatments need thermal insulators excellent in resistance to heat and heat insulation and unlikely to have the properties impaired at high temperatures. In view of the need, there are increased demands for thermal insulators prepared from carbon fibers and particularly formed thermal insulators which have a self-supporting ability needed for direct installation on the interior of a furnace without use of a support and which are easy to install on the furnace interior. It is also strongly desired to improve methods for molding such formed thermal insulators in order to reduce the production costs and to enhance the dimensional precision.
Feltlike carbon fibers useful as base materials for such formed thermal insulators (hereinafter referred to as "carbon fiber felt") have such a low bulk density that they need to be subjected to compression molding until they are given the desired bulk density required of thermal insulators.
In view of the above prior art problems, a method for producing formed thermal insulators was proposed which comprises impregnating a carbon fiber felt piece with a carbonizable resin, laminating the resin-impregnated felt piece under compression into a molded body of the desired thickness and bulk density and firing the molded body (Japanese Examined Patent Publication No. 35930/1975). The disclosed prior art includes a method for producing a formed thermal insulator having the desired bulk density which comprises winding around a mandrel a felt piece impregnated with a carbonizable resin, externally enclosing the winding with a metal foil, further binding it with a metal band for compression and curing and carbonizing the resin in the felt piece being compressed.
Another method proposed for producing a formed multi-layered thermal insulator for a vacuum furnace comprises adhering a carbon fiber felt piece to a graphite sheet of high density with a carbonaceous binder (Japanese Examined Utility Model Publication No. 29129/1983).
However, the method disclosed in Japanese Examined Patent Publication No. 35930/1975 has drawbacks. While the method is capable of adjusting the bulk density of the felt by control of the bulk compressibility and can produce a formed insulation material of excellent heat insulation property, the compression applied in the method can not uniformly act on the molded product therethroughout, inducing a pressure distribution between the surface layer and the internal layer portions of the molded product and an uneven bulk density distribution. The method also entails difficulties in controlling the bulk density distribution and producing a formed thermal insulator of excellent heat insulation property. Moreover, because of a low tension applicable to the felt piece, the method requires external clamping of the laminated felt piece with a metal band after externally enclosing the laminated felt with a metal foil, which results in the disarray of the spiral winding form into numerous wavelike deformities in the surface and internal portions of the thermal insulator. Thus the method fails to obtain molded bodies outstanding in surface smoothness and uniformity and having a good appearance. The method has a further problem of involving numerous steps and thus being low in productivity.
Japanese Examined Utility Model Publication No. 29179/1983 discloses a thermal insulator composed of a molded carbon fiber felt piece having graphite sheets attached thereto in the surface and intermediate portions. The disclosed thermal insulator is allegedly capable of preventing the damage to the material which would occur due to the penetration of molten metal into the material when used as a thermal insulator for a high-temperature vacuum melting furnace. Yet the bulk density of this thermal insulator is substantially equal to that of conventional thermal insulators and is not variable in the direction of the thickness. The thermal insulator has another problem that graphite is expensive.